This invention relates to cryogenic freezers, and, more particularly, to a vacuum insulated cryogenic freezer that provides increased storage capacity.
Cryogenic freezers have a wide variety of industrial applications, including but not limited to, storing biological materials such as blood, bone marrow, and micro-organic cultures. These biological materials must be maintained at low temperatures in order to be stored for an extended period without deteriorating.
Cryogenic freezers are double walled, vacuum insulated containers partially filled with a cryogenic liquid such as liquid nitrogen for establishing an extremely cold storage environment. Liquid nitrogen has a low boiling point, 77.4 K (-320.4.degree. F). Since cryogenic liquids have a low boiling point and, thus, a low heat of vaporization, heat inflow from the ambient can cause significant losses of cryogen due to the evaporation.
In order to minimize the amount of cryogen lost due to evaporation, the cryogenic freezer requires thermal and radiant barriers such as insulation and a high vacuum between the container walls. The vacuum space can also be filled with multiple layers of insulation to reduce heat transfer.
An example of multi-layered insulation is a low conductive sheet material comprised of fibers for reducing heat transfer by conduction. Also, the insulation can comprise radiation layers that are combined with the fiber layers. The radiation layer reduces the transmission of radiant heat in the freezer see for example U.S. Pat. No. 5,542,255 to Preston et al. and U.S. Pat. No. 5,404,918 to Gustafson.
The insulation and vacuum chamber of prior cryogenic freezers addresses the heat transfer problems due to the low boiling point of the cryogen. But, the characteristics of the insulation materials pose limitations to the physical design of the cryogenic freezers.
Containers have been designed with the vacuum space capable of maintaining a low pressure of 0.1 microns when the container is holding a cryogen. The shape of these containers has been restricted to a round, oval, or cylindrical structure. These structure provide the strength required by the walls of the container when such a high vacuum is drawn. If the cryogenic freezer were rectangular, the walls would collapse or deform when the vacuum is drawn due to insufficient structural support. Typically, the insulation materials disposed in the vacuum space of flat panel freezers fail to provide enough structural support for the container walls. Thus, the shape of the container is limited to cylindrical shapes.
Accordingly, it is desirable to provide a cryogenic freezer with optimum storage capacity such as a cube or rectangular enclosure which enables the walls of the freezer to maintain their shape when a high vacuum is drawn.
It is an object of the present invention to provide a cryogenic freezer that offers maximum storage capability at a low cost with flat interior and exterior walls.
It is another object of this invention to provide a cryogenic freezer with minimal thermal conductivity.
It is another object of the invention to provide a cryogenic freezer with reduced radiant energy transfer.